There are available position and speed sensors that include absolute and incremental digital encoders, resolvers based on the varying mutual inductances of windings moved relative to one another, variable-capacitance devices, hall devices sensing permanent-magnet fields, and inductosyn type transducers.
For some applications, however, these solutions for position sensing may be too costly or unreliable in hostile environments, particularly where the application requires detection of only one specific angular or linear position.
A more recent invention in position sensors employs the screened magnetic coupling technique. According to this technique, a properly dimensioned conductive screen is attached to a rotating or linearly moving object such that the screen can pass through an air gap between at least two pairs of magnetically coupled coils. When the screen moves into the air gap, it blocks the magnetic coupling between the coils thereby reducing the induced voltage in at least one of the coils. From this varying voltage, the position of the screen can be determined.
A position sensor utilizing this technique that provides a continuous stream of analog or digitally coded position signals is shown in U.S. Pat. No. 4,737,698, entitled "Position and Speed Sensors", issued on Apr. 12, 1988.